How does your brain create a picture?

When you look at a scene, each of the different 'seeing' areas in your brain seems to have a 'map' of the scene to which it adds details - like movement, colour, depth or shape. Scientists have learnt a lot about how you see by studying patients who have damage to these areas. Damage to any area can mean that the final picture is missing a particular detail.

What is it?

Many of the neurons in the visual part of the brain respond specifically to edges orientated in a certain direction. From this, the brain builds up the shape of an object. Information about the features on the surface of an object, like colour and shading, provide further clues about its identity. Objects are probably recognised mostly by their edges, and faces by their surface features.

Where is it?

When you look at an object, each of your eyes sees a slightly different picture. These signals are brought together in the brain, to help tell how far away an object is. This is what enables us to see 'magic eye' pictures. Other clues like shadows, textures and prior knowledge also help us to judge depth and distance.

Is it moving?

When you look at a moving object, signals go to a special part of your brain. Damage to this area can stop you seeing movement, even though your sight is otherwise normal. One woman, who suffered such damage through a stroke, described what it was like. If she poured out a cup of tea, it appeared frozen in mid-air, like ice. When walking down the street, she saw cars and trams change position, but not actually move.

If you lost your ability to see movement, the traffic might appear frozen in time.

What is agnosia?

People with damage to certain areas of the brain can develop agnosia. A man with agnosia described a rose as 'about six inches in length, a convoluted red form with a linear green attachment', and a glove as 'a continuous surface infolded on itself, it appears to have five outpouchings'. He could neither name the objects nor recognise what they were used for. Occasionally, agnosia is limited to failure to recognise faces. In one case, a farmer was unable to recognise his friends and family, but had no problems identifying his sheep!

In one case of agnosia, a farmer was unable to recognise his friends and family, but could still identify his sheep!

How do you recognise faces?

When you try to recognise an unfamiliar face, you look for several things such as gender, age and race. People are very good at deciding whether a face is male or female, even when obvious clues – such as make-up and hairstyle – are missing. This judgement relies on many features, including thickness of eyebrows and how much the nose sticks out, both of which are more pronounced in men.